This invention relates to signal interpretation for selective signal receivers for so called key pulsing, a form of pushbutton calling using multifrequency (MF) tones. It is particularly intended to be used in connection with telephone systems using multifrequency tone keying and having receivers coded in a 2 .times. 1-out-of-4 signal code, and code converters converting said code into another code, such as BCD code.
Systems of this nature are best known as Touch Tone dialing, which is a proprietory term of the Bell System.
In pushbutton dialing systems receivers must detect the frequencies transmitted by the subscriber stations, store the same, and convert the same into d-c signals capable of being processed by central stations. In addition thereto, the aforementioned receivers must be capable of distinguishing with an utmost degree of certainty dialing signals in the voice frequency range from speech signals, and from disturbances.
The requirements imposed upon the circuitry which tests the multifrequency signals converted into direct-current pulses, changes the code thereof if necessary, or desirable, and transmits the called numbers to a storage register in the office vary widely.
For best protection of dialing signals against signals resulting from speech the signal interpreting circuitry must test whether or not two, and only two, frequencies are transmitted uninterruptedly during a predetermined period of time. Another requirement consists in that dialing signals of long duration are not twice evaluated, even in case that such signals are interrupted for short times, e.g. by a fault condition. A further requirement consists in that the point of time at which a particular dialing key is released be detected with a high degree of certainty, even in case of short changes of the voltage level of the line, due to some disturbances. The duration of the incoming dialing signals depends upon the duration the dialing keys are depressed, and thus subject to large variations. The shortest signals transmitted from a pushbutton dialing subscriber must be detected with certainty by the voice frequency receiver, yet in the interest of protection against speech-generated signals the receivers must have relatively extended periods available for evaluating the signals which are received by them. The duration of signals at the output of the receiver must last from a few milliseconds to several hundred milliseconds, depending upon the time the dialing keys are depressed. In order to achieve safe transmission even if the time of the operation of dialing keys is very short, it is desirable to extend the period during which the signal interpreting circuitry has an output beyond the period of actuation of the respective dialing key. Key dialing receivers are often centrally located, and are related to the calling subscriber and to the storage register of the telephone system only as the need arises. Therefore, the number of the outgoing lines of the signal interpreting circuitry should be minimized. It is desirable not to provide 8 outgoing lines which correspond to the 8 voice frequencies, or to provide 16 outgoing lines of which each corresponds to one of the 16 possible dialing signals, but to provide the signal interpreting unit with a converter for converting the dialing signals into BCD(binary coded decimal) signals, thus allowing to limit the number of outgoing lines to 4.
The technology of MF key dialing is well known in the art, in particular by the recommendations of the CCITT (International Telephone and Telephone Consultive Committee of the International Telecommunications Union). Hence the MF key dialing technology will be discussed hereinafter only to such extent as required for a complete understanding of the dial signal interpreting circuitry embodying the present invention. MF key dialing transmitters produce upon operation of each of the keys of a keyboard simultaneously two frequencies in the audible range. Each of these two frequencies pertains to one of two bands of frequencies, each such band including four frequencies. The MF receivers separate the low-band frequencies and the high-band frequencies by band-pass filters and by band-rejection or band-stop filters, and supply the separated frequencies to four tuned or selective receivers. A code tester ascertains that in transmitting a dial number one and only one frequency of each band of frequencies is simultaneously involved. Both frequencies must be received uninterruptedly for a predetermined period of time. If this protective period of time is not maintained, the receiver is reset and the transmission of dialing signals does not take place. This precludes transmission to the central station or exchange of dial-signal-simulating disturbances as, for instance, speech, switching of click noise, etc. In some systems resetting also occurs when noise frequencies occur in addition to the useful dialing frequencies tending to impair a correct evaluation or interpretation of the dial tones which are received.
Prior art MF dialing systems attempted with limited success to eliminate the effects of disturbances upon the proper transmission of dial information. The present invention is an improvement over prior art MF dialing systems and its main object is to preclude, or reduce, multiple interpretation and/or suppression of dial numbers. Short time reductions of the line voltage have little or no effect upon the effectiveness of the system.